Although fiber-based cigarette filter elements are well known and have been used for a number of years, the choice of components for this class of filters has remained quite limited over the years, due to cost factors and lack of general suitability of many natural fibers for high speed filter production using state of the art filter rod-making apparatus. In addition, the demands on present day commercial cigarette filter elements tend to conflict with respect to characteristics such as general filtration efficiency, selective filtration, draw, and filter element hardness.
While various synthetic fibers and fiber mixtures have been tried and evaluated, a substantial number of cigarette filter elements continue to favor old technology using cellulose acetate-based fiber, because of certain cost and handling advantages. For example, cellulose acetate tow can be processed into cuttable filter rods using an essentially unmodified state-of-the-art filter rod-making apparatus without serious jamming problems. This advantage is enjoyed despite present day need for substantial amounts of additives, including non-volatile liquid organic plasticizers such as triacetin, diacetin, citric acid, as well as lubricants, flavors, medicines, and selective filtering agents and the like. Generally, such additives are applied as aqueous solutions onto opened cellulose acetate fiber tow by dipping, spraying, and printing. In the case of plasticizer additives, the resulting softened areas are capable of randomly adhering to adjacent crimped fiber to impart some degree of rigidity or hardness to the resulting plug and filter rod, permitting subsequent cutting into filter element length.
The above-stated advantages of cellulose acetate fiber, however, are countered by certain serious disadvantages. For example, such fibers tend to be relatively weak (1.0-1.2 g./denier) compared with synthetics such as polyolefin fiber. This characteristic seriously limits the amount of tension and crimp that acellulose acetate fiber tow of low dpf fiber or filament will tolerate prior to introduction into a conventional filter rod-making apparatus.
Synthetic fiber components, particularly polyolefins such as polypropylene staple, offer a valuable alternative since they are easily drawn to a small denier and offer potentially high filter efficiency without significant loss of the strength needed for crimping and the tension of high speed production.
Polyolefin fibers, however, also have some disadvantages. These generally arise from the fact that polyolefin-containing substrates are generally hydrophobic and tend to be chemically inert, while a majority of known potential additives tend to be somewhat hydrophilic and difficult to retain in proper amount and in functional condition within filter elements composed of hydrophobic synthetic fiber.
Another substantial problem, unique to the cigarette filter art, concerns the difficulty in optimizing fiber denier and general filter efficiency of synthetic fiber filters without corresponding sacrifice in dimensional stability, hardness, and draw (resistance to draw) across the filter element. In particular, polyolefin-containing substrates (e.g. open tows and slivers) fed into a filter rod-making apparatus, demonstrate significant negative correlation between draw and hardness of the filter element. In addition, it is sometimes difficult to avoid jamming of heavily impregnated synthetic substrates fed at high speed into a conventional filter rod-making apparatus, particularly where the additives include substantial amounts of the above-noted modifier components and the like dispersed in viscous carriers or vehicles. The jamming problem is believed due, at least in part, to deficiencies in lubricating properties of such additives.
It is an object of the present invention to economically produce cigarette filters having superior efficiency.
It is a further object of the present invention to more effectively and selectively control the delivery of byproducts found in cigarette smoke, particularly tar, nicotine, formaldehyde and total particulate matter.